MO diagram relevant for the description of the five lowest-lying states of neutral allyl and the ground state of . The configurations on the right-hand side are labeled in the molecular symmetry group and, in each case, the CI coefficient of the dominant contribution to the electronic wave function is indicated as obtained in Ref. 21. Each orbital is given a symmetry label reflecting its dominant single-center expansion contribution, where and represent the quantum numbers associated with the orbital angular momentum and its projection onto the -axis of the molecule-fixed axis system. The , , , and orbitals are Rydberg orbitals, whereas the HOMO, SOMO, and LUMO are valence orbitals. A schematic representation of allyl and the definition of the principal and inertial axis systems are given at the bottom left.
Simulations and PFI-ZEKE photoelectron spectra of the origin band of allyl recorded through selected intermediate levels. The simulations were performed on the basis of the orbital ionization model assuming that ionization occurs out of pure (a) , (b) , (c) , and (d) Rydberg orbitals. In each trace only transitions from rovibronic states with were taken into account. PFI-ZEKE photoelectron spectra of the origin band of allyl recorded via (e) the level in two-photon excitation and (f) via the in one-photon excitation are displayed. Each spectrum has been shifted such that the horizontal axis corresponds to the absolute energy above the neutral vibronic ground state. See text for more details.
(a) Photoionization (top trace) and PFI-ZEKE photoelectron (middle trace) spectra of allyl in the region of the first adiabatic ionization threshold. A simulation of the spectrum based on the orbital ionization model and assuming a rotational temperature of 25 K is displayed on the bottom trace and has been shifted along the vertical axis for clarity. (b) Upper trace: PFI-ZEKE photoelectron spectrum of the transition of allyl. Bottom trace: The simulation based on the orbital ionization model and assuming a rotational temperature of 35 K. The simulated spectrum has been shifted along the vertical axis for clarity.
The REMPI (upper trace) and REMPI (lower trace) spectra of the allyl radical. The vertical scale has been chosen so that the maxima of both spectra have the same amplitude and do not reflect the relative photoionization signals.
The and PFI-ZEKE photoelectron spectra obtained via different intermediate states corrected for the thermal vibrational energy in the electronic ground state. The vertical scale of the different spectra has been chosen such that the maxima of all spectra have the same amplitude.
Rovibronic symmetry selection rules for transitions from vibronic states of of symmetry to the cationic vibronic ground state in dependence of the angular momentum of the ejected photoelectron . The last column lists the states of neutral allyl of the corresponding electronic symmetry (i.e., ).
First adiabatic ionization energy , rigid-rotor asymmetric-top rotational constants, and fundamental wave number of the C–C–C bending mode of and used in the simulations of the experimental spectra. Selected values of the and from the literature are given for comparison.
Transition wave numbers in and assignments of the intermediate states observed in the REMPI spectra of allyl and used as intermediate levels to record the PFI-ZEKE photoelectron spectra.
Experimental cation vibrational frequencies observed in the multiphoton PFI-ZEKE photoelectron spectra compared to anharmonic vibrational frequencies obtained at the HCTH147/TZ2P level of theory. The uncertainty in the experimental wave numbers is estimated to be .
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